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© 2007 Levente Buttyán Security and Privacy in Upcoming Wireless Networks Key establishment in ad hoc networks exploiting - physical contact; - mobility and vicinity;

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 2/46 Exploiting physical contact target scenarios –modern home with multiple remotely controlled devices DVD, VHS, HiFi, doors, air condition, lights, alarm, … –modern hospital mobile personal assistants and medical devices, such as thermometers, blood pressure meters, … common in these scenarios –transient associations between devices –physical contact is possible for initialization purposes the resurrecting duckling security policy –at the beginning, each device has an empty soul –each empty device accepts the first device to which it is physically connected as its master (imprinting) –during the physical contact, a device key is established –the master uses the device key to execute commands on the device, including the suicide command –after suicide, the device returns to its empty state and it is ready to be imprinted again Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 3/46 Does mobility increase or reduce security ? Mobility is usually perceived as a major security challenge in networking –Wireless communications –Sporadic availability of the user/node –Higher vulnerability of the device –Reduced computing capability of the devices However, in real life, people often move (and gather) to increase security –Face to face meetings –Transport of assets and documents –Authentication by physical presence Can we take advantage of mobility to increase security in networking? Yes, we can, assuming that –nodes are operated by humans –when in the vicinity of each other, nodes can use a secure side channel (e.g., infra red) to exchange a key –each node has some friends (peers that are trusted by the node), and there is already a key shared between each pair of friends Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 4/46 Infrared link (Alice, XYZ, key material) (Bob, UVW, key material) Visual recognition, conscious establishment of a two-way security association Secure side channel - Typically short distance (a few meters) - Ensures integrity and confidentiality Alice Bob Exploiting vicinity and the secure side channel Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 5/46 Colin (common friend of Alice and Bob) Bob (Colin’s friend) Alice Taking advantage of common friends trust and already established key trust and already established key identifiers and key material Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 6/46 What if there’s no common friend? Key establishment in ad hoc networks Colin (a friend Bob) Bob (Colin’s friend) Alice trust and already established key key established via the secure side channel key material David (a friend of Alice) trust and already established key key established via the secure side channel key material

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 7/46 A possible implementation notes: –single trusted party is replaced with two parties trusted by one entity each –if f and g are not colluding, then they cannot compute kuv –both u and v trust at least one of f and g for not colluding Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 8/46 Depends on several factors: – Area size – Number of communication partners: s – Number of nodes: n – Number of friends – Mobility model and its parameters (speed, pause times, …) Established security associations : e ij (t) = 1, if at time t nodes i and j already share a key, and 0 otherwise Desired security associations : p ij = 1, if i and j wants to setup a shared key, and 0 otherwise Convergence : Pace of establishment of the security associations Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 9/46 Random walk – discrete time – simple, symmetric random walk – area: Bounded and toroid grids (33x33, 100x100, 333x333) – number of nodes: 100 Random waypoint – most commonly used in mobile ad hoc networks – continuous time – area size: 1000m x1000m – max speed: 5m/s, 20m/s – pause time: 5s, 100s, 200s – security power range: 5m (SSC), 50m 100m (radio) Common simulation settings – simulations are run 20 times – confidence interval: 95% p=1/5 Mobility models Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 10/46 8 Any point on the plane Restricts the movement of nodes to a set of points with a predefined probability Regular random waypoint is a special case ( = 0) area size: 1000m x1000 m max speed: 5m/s, 20m/s pause time: 5s, 100s, 200s restriction probability: 0.1, 0.5, 1 number of restriction points: 20 (Restricted) random waypoint Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 11/46 Size matters t M =O(NlogN) Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 12/46 Friends help Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 13/46 Security range matters Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 14/46 Meeting points help Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 15/46 Pause time Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 16/46 Can the secure side channel be based on radio? Diffie-Hellman with authentication of messages by the human operator of the devices –Alice and Bob comes together –their devices execute the basic Diffie-Hellman protocol over the radio channel –Alice’s device hashes the concatenation of all messages that it sent and that it received in the protocol –Bob’s device hashes the concatenation of all messages that it received and that it sent in the protocol –the two hash must match –both devices display the hash values in some human readable format select a set of worlds from a common dictionary based on the hash value visualize the hash function with a Random Art image –the human operators compare the displayed hash value Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 17/46 Application of Random Art images the hash value is converted into three two-variable functions –each can be represented as a tree these formulae define the color value of each pixel of the resulting image + sinsqrt *rnd+ sqrtx y rnd y Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 18/46 Can the secure side channel be based on radio? Diffie-Hellman with I-codes –special PHY layer that uses on-off keying (presence of signal means 1, absence of signal means 0) –active adversary can turn a 0 into a 1, but not vice versa –each message sent is encoded in such a way that it contains the same number of 0s and 1s (I-codes) –messages cannot be modified in an unnoticeable way –encoding messages with I-codes ensures the integrity of the communications Man-in-the-Middle is excluded Key establishment in ad hoc networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 19/46 Summary it is possible to establish pairwise shared keys in ad hoc networks without a globally trusted third party mobility, secure side channels, and friends are helpful the pace of establishment of the security associations is strongly influenced by the area size, the number of friends, and the speed of the nodes the proposed solution can easily be implemented with both symmetric and asymmetric crypto Key establishment in ad hoc networks

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© 2007 Levente Buttyán Security and Privacy in Upcoming Wireless Networks Key establishment in sensor networks key types; establishment of link keys using a short-term master key; random key pre- distribution: - the basic scheme, and - some improvements;

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 21/46 Key establishment in sensor networks due to resource constraints, asymmetric key cryptography should be avoided in sensor networks we aim at setting up symmetric keys requirements for key establishment depend on –communication patterns to be supported unicast local broadcast global broadcast –need for supporting in-network processing –need to allow passive participation useful key types –node keys – shared by a node and the base station –link keys – pairwise keys shared by neighbors –cluster keys – shared by a node and all its neighbors –network key – a key shared by all nodes and the base station Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 22/46 Setting up node, cluster, and network keys node key –can be preloaded into the node before deployment cluster key –can be generated by the node and sent to each neighbor individually protected by the link key shared with that neighbor network key –can also be preloaded in the nodes before deployment –needs to be refreshed from time to time (due to the possibility of node compromise) neighbors of compromised nodes generate new cluster keys the new cluster keys are distributed to the non-compromised neighbors the base station generates a new network key the new network key is distributed in a hop-by-hop manner protected with the cluster keys Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 23/46 Design constraints for link key establishment network lifetime –severe constraints on energy consumption hardware limits –8-bit CPU, small memory –large integer arithmetics are infeasible no tamper resistance –nodes can be compromised –secrets can be leaked no a priori knowledge of post-deployment topology –it is not known a priori who will be neighbors gradual deployment –need to add new sensors after deployment Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 24/46 Traditional approaches use of public key crypto (e.g., Diffie-Hellman ) –limited computational and energy resources of sensors use of a trusted key distribution server (Kerberos-like) –base station could play the role of the server –requires routing of key establishment messages to and from the base station routing may already need link keys unequal communication load on the sensors –base station becomes single point of failure pre-loaded link keys in sensors –post-deployment topology is unknown –single “mission key” approach vulnerable to single node compromise –n -1 keys in each of the n sensors excessive memory requirements gradual deployment is difficult doesn’t scale Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 25/46 Link key setup using a short-term master key (LEAP) main assumptions: –any sensor node deployed will not be compromised within T min time –any node can discover its neighbors and set up neighbor relationships within T est < T min time typically, T est is a few seconds, so these assumptions make sense in practice protocol: –key pre-distribution phase –neighbor discovery phase –link key establishment phase –key erasure phase Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 26/46 Link key setup using a short-term master key (LEAP) key pre-distribution phase –before deployment, each node is loaded with K I –each node u derives a node key K u as K u = f(K I, u), where f is a one-way function neighbor discovery phase –when a node deployed, it tries to discover its neighbors by broadcasting a HELLO message u *: u, N u where N u is a random nonce –each neighbor v replies with v u: v, mac(K v, v|N u ) –u can compute f(K I, v) = K v, and verify the authenticity of the reply Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 27/46 Link key setup using a short-term master key (LEAP) link key establishment phase –u computes the link key K uv = f(K v, u) –v computes the same key –no messages are exchanged –note: u does not authenticate itself to v, but … only a node that knows K I can compute K uv a compromised node that tries to impersonate u cannot know K I (see below) key erasure phase –T min time after its deployment, each node deletes K I and all keys it computed in the neighbor discovery phase Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 28/46 Random key pre-distribution – Preliminaries Given a set S of k elements, we randomly choose two subsets S 1 and S 2 of m 1 and m 2 elements, respectively, from S. The probability of S 1 S 2 is Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 29/46 The basic random key pre-distribution scheme initialization phase –a large pool S of unique keys are picked at random –for each node, m keys are selected randomly from S and pre-loaded in the node (key ring) direct key establishment phase –after deployment, each node finds out with which of its neighbors it shares a key (e.g., each node may broadcast the list of its key IDs) –two nodes that discover that they share a key verify that they both actually posses the key (e.g., execute a challenge-response protocol) path key establishment phase –neighboring nodes that do not have a common key in their key rings establish a shared key through a path of intermediaries –each link of the path is secured in the direct key establishment phase Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 30/46 Setting the parameters connectivity of the graph resulting after the direct key establishment phase is crucial a result from random graph theory [Erdős-Rényi]: in order for a random graph to be connected with probability c (e.g., c = 0.9999), the expected degree d of the vertices should be: (1) in our case, d = pn’ (2), where p is the probability that two nodes have a common key in their key rings, and n’ is the expected number of neighbors (for a given deployment density) p depends on the size k of the pool and the size m of the key ring (3) c d p k, m (1)(2)(3) Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 31/46 Setting the parameters – an example number of nodes: n = 10000 expected number of neighbors: n’ = 40 required probability of connectivity after direct key establishment: c = 0.9999 using (1): required node degree after direct key establishment: d = 18.42 using (2): required probability of sharing a key: p = 0.46 using (3): appropriate key pool and key ring sizes: k = 100000, m = 250 k = 10000, m = 75 … Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 32/46 Qualitative analysis advantages: –parameters can be adopted to special requirements –no need for intensive computation –path key establishment have some overhead … decryption and re-encryption at intermediate nodes communication overhead –but simulation results show that paths are not very long (2-3 hops) –no assumption on topology –easy addition of new nodes disadvantages: –node capture affects the security of non-captured nodes too if a node is captured, then its keys are compromised these keys may be used by other nodes too –if a path key is established through captured nodes, then the path key is compromised –no authentication is provided Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 33/46 Improvements: q-composite rand key pre-distribution basic idea: –two nodes can set up a shared key if they have at least q common keys in their key rings –the pairwise key is computed as the hash of all common keys advantage: –in order to compromise a link key, all keys that have been hashed together must be compromised disadvantage: –probability of being able to establish a shared key directly is smaller (it is less likely to have q common keys, than to have one) –key ring size should be increased (but: memory constraints) or key pool size should be decreased (but: effect of captured nodes) Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 34/46 q-composite scheme: Simulation results m = 200, p = 0.33 taken from: H. Chan and A. Perrig and D. Song, "Random key predistribution schemes for sensor networks", IEEE Security and Privacy Symp. (Oakland), 2003 Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 35/46 Improvements: Multipath key reinforcement basic idea: –establish link keys through multiple disjoint paths –assume two nodes have a common key K in their key rings –one of the nodes sends key shares k 1, …, k j to the other through j disjoint paths –the key shares are protected during transit by keys that have been discovered in the direct key establishment phase –the link key is computed as K + k 1 + … + k j radio connectivityshared key connectivity k2k2 K multipath key reinforcement Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 36/46 Improvements: Multipath key reinforcement advantages: –in order to compromise a link key, at least one link on each path must be compromised increased resilience to node capture disadvantages: –increased overhead note: –multipath key reinforcement can be used for path key setup too Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 37/46 Multipath scheme: Simulation results m = 200, p = 0.33 Key establishment in sensor networks taken from: H. Chan and A. Perrig and D. Song, "Random key predistribution schemes for sensor networks", IEEE Security and Privacy Symp. (Oakland), 2003

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 38/46 Polynomial based key pre-distribution let f be a bivariate t-degree polynomial over a finite field GF(q), where q is a large prime number, such that f(x, y) = f(y, x) each node is pre-loaded with a polynomial share f(i, y), where i is the ID of the node any two nodes i and j can compute a shared key by –i evaluating f(i, y) at point j and obtaining f(i, j), and –j evaluating f(j, y) at point i and obtaining f(j, i) = f(i, j) this scheme is unconditionally secure and t-collusion resistant –any coalition of at most t compromised nodes knows nothing about the shared keys computed by any pair of non-compromised nodes any pair of nodes can establish a shared key without communication overhead (if they know each other’s ID) memory requirement of the nodes is (t +1) log(q) problem: t is limited by the memory constraints of the sensors Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 39/46 Polynomial based random key pre-distribution operation: –let S be a pool of bivariate t-degree polynomials –for each node i, we pick a subset of m polynomials from the pool –we pre-load into node i the polynomial shares of these m polynomials computed at point i –two nodes that have polynomial shares of the same polynomial f can establish a shared key f(i, j) –if two nodes have no common polynomials, they can establish a shared key through a path of intermediaries advantage: –can tolerate the capture of much more than t nodes (t can be smaller, but each node needs to store m polynomials) in order to compromise a polynomial, the adversary needs to obtain t + 1 shares of that polynomial it is very unlikely that t + 1 randomly captured nodes have all selected the same polynomial from the pool Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 40/46 Simulation results m = 200, p = 0.33 taken from D. Liu and P. Ning, “Establishing pairwise keys in distributed sensor networks", ACM CCS, 2003. Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 41/46 Matrix based key pre-distribution (Blom’s scheme) let G be a (t + 1)×n matrix over a finite field GF(q) (where n is the size of the network) let D be a random (t +1)×(t +1) symmetric matrix over GF(q) G is public, D is secret let A = (DG) T and K = AG –K is a symmetric matrix, because K = AG = (DG) T G = G T D T G = G T DG = G T A T = (AG) T = K T each node i stores the i-th row of A any two nodes i and j can compute a shared key K ij –i computes A(i,.)G(.,j) = K ij –j computes A(j,.)G(.,i) = K ji = K ij Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 42/46 Matrix based random key pre-distribution G is as before D 1, …, D k are random (t +1)×(t +1) symmetric matrices A v = (D v G) T and {A v } is the pool (of spaces) for each node i, we pick a random subset of the pool and pre-load in the node the i-th row of the selected matrices (i.e., A v (i,.) for each selected v) if two nodes i and j both selected a common matrix A v, then they can compute a shared key using Blom’s scheme if two nodes don’t have a common space, they can setup a key through intermediaries Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 43/46 Simulation results m = 200, p = 0.33 taken from W. Du and J. Deng and Y. S. Han and P. K. Varshney, "A pairwise key pre-distribution scheme for wireless sensor networks", ACM CCS, 2003 Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 44/46 Assumptions revisited (and more) [DiPietro et al.] the connectivity graph resulting after the first phase of the basic protocol is not a random graph (in the sense of Erdős- Rényi) –an edge between two nodes exists if they have at least one common key AND they are in power range of each other –the Erdős-Rényi theorem may not hold for these types of graphs almost sure connectivity after the first phase of the basic protocol is ensured if –m 2 / k = c log(n) / n –k > n-1 –m > 4 Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 45/46 Assumptions revisited (and more) [DiPietro et al.] a definition of security: a network is redoubtable, if the probability that compromising a sub- linear fraction of nodes (uniformly chosen at random) can compromise a constant fraction of links in the network is 0 a network is redoubtable if m/k ~ 1/n a practical choice of parameters: –m = log(n) –k = n log(n) Key establishment in sensor networks

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Security and Privacy in Upcoming Wireless Networks SWING’07, Bertinoro, Italy, 2007. 46/46 Summary in sensor networks, we need different types of keys node keys, cluster keys, and network keys can be established relatively easily using the technique of key pre- loading and using already established link keys link keys can be established using a short-term master key or with the technique of random key pre-distribution Key establishment in sensor networks

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